A common type of drive system using plural motors couples each of the motors to a load being driven through the drive shafts of the motors and means such as gear boxes and/or line shafts. In such drive systems, the motor loads may become unstable and fail to equally share the load resulting in the inability of the system to utilize the full load capacity of all the motors and impart undue stress to the drive train. In overhead traveling cranes and gantry cranes having an overhead lifting beam, plural drive systems comprising two motors independently connected to a common hoist rope drum are sometimes a preferred drive method. Such overhead cranes travel on a pair of elevated rails which are parallel and spaced apart. One or more bridge girders span the rails and have drive wheels mounted at either end in engagement with the rails to move the girders and thereby the crane along the rails. In gantry cranes, parallel spaced apart rails are positioned at ground level and legs connected at their upper ends by load carrying beams or girders are supported at their lower ends by drive wheels mounted on the legs in engagement with the wheels. The wheels are driven to thereby move the gantry crane along the parallel rails.
A trolley is mounted on parallel rails affixed to the overhead girders of the overhead or gantry crane and has drive wheels engaging the girder rails to move the trolley along the length of the girders. A load hoist is mounted on the trolley and includes a powered hoist rotatable drum about which a steel rope is wound or unwound to lift or lower a load. The rope is connected to a load lifting device such as a hook, sling or a magnet. The lifting or lowering operation of the hoist, the movement of the trolley on the overhead girders, and the movement of the crane along its rails are controlled by an operator to pick up, move and deposit a load anywhere in the area traveled by the crane.
Drive systems for cranes have increasingly used alternating current motors provided with adjustable frequency power supplies. Adjustable frequency power supplies can be controlled by the operator to provide a frequency to a motor ranging from zero to rated frequency or more. Squirrel cage induction type motors are typically used and are reliable, low cost and widely available. In an adjustable frequency power supply, not only can the frequency be varied, but the voltage supplied to the motor and, due to both the ability to vary the frequency and the voltage, the motor current and thereby the motor output torque can be controlled. More recently, adjustable frequency power supplies utilizing angular positions of current vectors to control motor output torque have come into use.
It is sometimes desirable to provide overhead and gantry type cranes, particularly the hoists of such cranes, with two motors both having their drive shaft output ends mechanically coupled to and driving the same hoist drum through suitable gear boxes. To assist in maintaining an equal division of the hoist load between the motors, the drive shafts of the motors are also coupled directly to each other. Each motor is provided with a separate adjustable frequency power supply. A single master switch moved by the operator controls the frequency output of both adjustable frequency power supplies to the two motors. Each motor will draw motor current from its connected power supply and provide output torque to the hoist drum to approximately carry one half of the load of the hoist drum. If the power supplies to the motors are of the type that utilize current vector positions to maximize the torque provided by each motor, an encoder having its output fed back to the power supply of a first one of the motors is provided for sensing the speed of the first motor output shaft and its rotor, and the rotor position, which can be used to calculate the angular position of the rotor load current vector. The first motor is considered as the master motor. A sensor is also provided for each motor for indicating the angular position of the stator flux current vector in each motor. The maximum torque of each motor is produced when its rotor load current and stator flux current vectors are spaced at a ninety degree angle to each other and, upon the comparison of the angular difference of the encoder output and the sensor output, if the angular difference is not ninety degrees, the magnitude of the stator current of the power supply to the motor will be changed to result in a shift in the position of the stator flux current vector to produce the desired ninety degree relationship with the rotor load current vector.
Both motors are supplied with the same frequency so that they should rotate at the same speed and, at that speed, both should produce the same maximum torque due to the current vector angle control. However, in fact, the two motors do not operate stably to equally share the load. This is due largely to variation in fabrication of motors that are otherwise identical and differences in fit and friction of the drive train connected to the motor and hoist drum. The instant invention is an improvement in motor load sharing systems where plural motors are coupled to the same load.